The subject matter disclosed herein relates to a flow cell for a flow meter.
Flow meters, including ultrasonic flow meters, are used to determine the characteristics (e.g., flow rate, pressure, temperature, etc.) of fluids (e.g., liquids, gases, etc.) flowing in pipes of different sizes and shapes. Knowledge of these characteristics of the fluid can enable other physical properties or qualities of the fluid to be determined. For example, in some custody-transfer applications, the flow rate can be used to determine the volume of a fluid (e.g., oil or gas) being transferred from a seller to a buyer through a pipe to determine the costs for the transaction, where the volume is equal to the flow rate multiplied by the cross sectional area of the pipe.
In one type of ultrasonic flow meter employing transit time flow metering, one or more pairs of ultrasonic transducers can be installed in or on a flow cell, where each pair can contain transducers located upstream and downstream from each other forming an ultrasonic path between them at a particular chord location across the pipe. FIG. 1 illustrates an existing flow cell with a total of four ultrasonic paths formed by four pairs of upstream and downstream transducer assemblies (i.e., total of eight transducer assemblies—two for each ultrasonic path). Each transducer, when energized, transmits an ultrasonic signal (e.g., a sound wave) along an ultrasonic path through the flowing fluid that is received by and detected by the other transducer. The path velocity of the fluid averaged along the ultrasonic path at a particular chord location can be determined as a function of the differential between (i) the transit time of an ultrasonic signal traveling along the ultrasonic path from the downstream transducer upstream to the upstream transducer against the fluid flow direction, and (2) the transit time of an ultrasonic signal traveling along the ultrasonic path from the upstream transducer downstream to the downstream transducer with the fluid flow direction.
In an existing flow cell construction as shown in FIG. 1, nozzles are welded to the pipe of the flow cell forming ports for the installation of the transducer assemblies. If the corresponding pair of transducer assemblies are not properly aligned or if the ultrasonic path formed by the corresponding pair of transducer assemblies is not precisely located at the desired chord location, this welding process, which can have significant mechanical tolerances, can lead to inaccurate flow rate measurements. In order to provide more precise alignment of transducer assemblies and chord locations over flow cells using welded nozzles, other flow cells are manufactured by machining a forged block of material to form the flow cell and the sensor ports for the transducer assemblies. While this machining process can improve accuracy, the process can be time consuming and expensive.
Also, as can be seen in FIG. 1, in an existing flow cell construction, the transducer assemblies can extend well beyond the diameter of the flanges and the flange-to-flange distance of the flow cell and are connected to the flow meter by cables external to the flow cell. The presence of externally located transducer assemblies and associated cabling increases, e.g., the size, cost, complexity, maintenance, certifications, and time required for flow cell installations, requiring additional fittings, conduits, and junction boxes external to the flow cell. This construction also limits the number of transducer assemblies and sensor ports for the transducer assemblies that can be installed at one location on the flow cell. For example, as shown in FIG. 1, in order to provide four ultrasonic paths (which requires four corresponding pairs of transducer assemblies at four chord locations), since four transducer assemblies cannot be installed at one location on the flow cell (i.e., will not fit in a vertical stack), the sensor ports for the first two pairs of transducer assemblies are located opposite each other in a first plane, while the sensor ports for the second two pairs of transducer assemblies are located opposite each other in a second plane that intersects the first plane. This crisscross configuration results in having the upstream sensor ports for the transducer assemblies from the first two pairs of transducer assemblies upstream from the downstream sensor ports for the two transducer assemblies from the second two pairs of transducer assemblies (or vice versa) on the same side of the flow cell. The presence of these upstream sensor ports and transducer assemblies can cause port effects that can affect the accuracy of the flow rate measurements by the downstream transducer assemblies.
Furthermore, since flow cells are often installed in hazardous locations (e.g., refineries, chemical plants, etc.), the externally routed cables, junction boxes, fittings, covers, etc. need to be rated for use in a hazardous location with the parts and final installation complying with local requirements for such hazardous locations. Since the hazardous location requirements vary throughout the world (e.g., some jurisdictions allow armored cable, some require rigid conduit, some allow flexible conduit, etc.), the same flow cell and flow meter may need several different configurations and different parts depending on where the equipment is installed to comply with local requirements for installation in a hazardous location. For example, for a flow cell machined from a forged block of material with machined sensor ports, a single cover can be used to seal proximate sensor ports. Since the size of the cover will vary depending upon the number of sensor ports and the diameter of the flow cell (e.g., the cover for proximate sensor ports for a 10 in. (25.4 cm) diameter flow cell will be smaller than the cover for proximate sensor ports for a 36 in. (91.44 cm) diameter flow cell), several different covers are needed for the various sized flow cells and each must be certified for use in a hazardous location.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.